Field of the Invention
The present invention relates to an apparatus for the transmission of electric power produced by a multi-phase alternating current generator to a multi-phase alternating-current power network, said apparatus comprising a control unit, a non-controlled rectifier bridge, a direct-current intermediate circuit and a controlled inverter bridge for supplying direct-current power into the alternating-current power network.
Description of Background Art
In the production of electric power, e.g. in hydropower and wind power stations, synchronous generators are generally used for the conversion of mechanical energy into electric energy. When a synchronous generator is connected directly to an electric power network, its speed of rotation must be exactly in synchronism with the frequency of the electric network. Thus, a drawback with such an arrangement is this very need for precise adjustment of rotational speed, which e.g. in wind power stations requires laborious adjustment of the blade angle.
There are prior-art control apparatuses that improve the situation in that the rotational speed of the generator need not be exactly constant. In such apparatuses, the a.c. power produced by the generator is rectified, smoothed in a d.c. capacitor functioning as an intermediate energy storage and then fed via an electronic power inversion stage into the electric network. The rectifier may consist of a fully controlled inverter circuit producing a continuously constant d.c. voltage from the voltage generated by the generator, which may vary within a large range. The rectifier may also be a non-controlled diode bridge circuit.
The constant d.c. voltage of the intermediate circuit is converted into an a.c. voltage by means of a circuit consisting of an inverter and a choke and fed into the electric network. The inverter is typically a bridge circuit formed from fully gate-controlled power semiconductor switches with diodes connected in parallel with them, the power semiconductor switches being typically controlled at a frequency of a few kHz. The harmonics generated in the line current by high-frequency operation are suppressed by means of a high-inductance network inductor.
In the circuit, the power supplied into the electric network can be controlled by the amplitude of the voltage produced by the inverter and its phase difference angle relative to the voltage of the electric network. It is preferable to adjust the amplitude and phase difference angle so that the current and voltage of the supply network are cophasal, i.e. cosφ=1.
In a solution applying these technical principles, the aim is to maintain a constant d.c. intermediate-circuit voltage by using a high-capacitance d.c. capacitor. The capacitor functions as an intermediate energy storage, allowing the rectifying and inverting circuits to work independently of each other, which is an advantage, e.g., in respect of control engineering.
The ratings of the capacitor unit are generally determined by the capacitor's ability to withstand the current and voltage load imposed on them as well as the required service life in extreme conditions. To calculate the current load, the current components generated by the rectifying and inverting circuits are generally first calculated separately and then summed quadratically. This is the procedure followed when the capacitor unit has a notably high capacitance, in which case the circuits can be regarded as separate circuits and their instantaneous values have no effect on each other. From this premise it follows that the capacitance of the capacitor unit becomes very high because the advantageous capacitor type, the electrolytic capacitor, has a relatively low current tolerance.
Thus, the above-described circuit has the disadvantage of requiring a large d.c. capacitor and a large network inductor as well as a complex inverter bridge control logic. The voltage produced by the inverter consists of columns of a height corresponding to the d.c. voltage, and the generation of these requires high-frequency operation of the power semiconductors, which again involves considerable switching losses and a need for efficient cooling.
A previously known possibility of implementing the inverter is a three-phase rectifier bridge circuit as disclosed in patent specification U.S. Pat. No. 4,447,868, which allows the power to flow either from the a.c. circuit into the d.c. circuit or conversely. According to the aforesaid patent, when the power is flowing from the d.c. circuit into the supply network, the rectifier transistors are controlled in such manner that the control unit controls their conduction so that the upper branch transistor, in the phase in which the instantaneous value of the supply voltage is highest, and the lower branch transistor, in the phase in which the instantaneous value of the supply voltage is lowest, are conducting. The circuit described in the aforesaid specification also requires the use of a complex inverter control logic as well as a large inductance and a large capacitance in the d.c. circuit.
All the prior-art devices described above need capacitors having a large capacitance as an energy storage as well as current-limiting inductors with a voltage across them that typically may be about 3-5% of the supply voltage, the inductors thus having a large inductance. Such components are bulky and expensive. Therefore, they are a very significant factor affecting the size and costs of the apparatus.